On 23/12/2019 22:18, Michael Ellerman wrote:
> Alexey Kardashevskiy <a...@ozlabs.ru> writes:
>
>> The last jump to free_exit in mm_iommu_do_alloc() happens after page
>> pointers in struct mm_iommu_table_group_mem_t were already converted to
>> physical addresses. Thus calling put_page() on these physical addresses
>> will likely crash.
>>
>> This moves the loop which calculates the pageshift and converts page
>> struct pointers to physical addresses later after the point when
>> we cannot fail; thus eliminating the need to convert pointers back.
>>
>> Fixes: eb9d7a62c386 ("powerpc/mm_iommu: Fix potential deadlock")
>> Reported-by: Jan Kara <j...@suse.cz>
>> Signed-off-by: Alexey Kardashevskiy <a...@ozlabs.ru>
>> ---
>> Changes:
>> v3:
>> * move pointers conversion after the last possible failure point
>> ---
>> arch/powerpc/mm/book3s64/iommu_api.c | 39 +++++++++++++++-------------
>> 1 file changed, 21 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/book3s64/iommu_api.c
>> b/arch/powerpc/mm/book3s64/iommu_api.c
>> index 56cc84520577..ef164851738b 100644
>> --- a/arch/powerpc/mm/book3s64/iommu_api.c
>> +++ b/arch/powerpc/mm/book3s64/iommu_api.c
>> @@ -121,24 +121,6 @@ static long mm_iommu_do_alloc(struct mm_struct *mm,
>> unsigned long ua,
>> goto free_exit;
>> }
>>
>> - pageshift = PAGE_SHIFT;
>> - for (i = 0; i < entries; ++i) {
>> - struct page *page = mem->hpages[i];
>> -
>> - /*
>> - * Allow to use larger than 64k IOMMU pages. Only do that
>> - * if we are backed by hugetlb.
>> - */
>> - if ((mem->pageshift > PAGE_SHIFT) && PageHuge(page))
>> - pageshift = page_shift(compound_head(page));
>> - mem->pageshift = min(mem->pageshift, pageshift);
>> - /*
>> - * We don't need struct page reference any more, switch
>> - * to physical address.
>> - */
>> - mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
>> - }
>> -
>> good_exit:
>> atomic64_set(&mem->mapped, 1);
>> mem->used = 1;
>> @@ -158,6 +140,27 @@ static long mm_iommu_do_alloc(struct mm_struct *mm,
>> unsigned long ua,
>> }
>> }
>>
>> + if (mem->dev_hpa == MM_IOMMU_TABLE_INVALID_HPA) {
>
> Couldn't you avoid testing this again ...
>
>> + /*
>> + * Allow to use larger than 64k IOMMU pages. Only do that
>> + * if we are backed by hugetlb. Skip device memory as it is not
>> + * backed with page structs.
>> + */
>> + pageshift = PAGE_SHIFT;
>> + for (i = 0; i < entries; ++i) {
>
> ... by making this loop up to `pinned`.
>
> `pinned` is only incremented in the loop that does the GUP, and there's
> a check that pinned == entries after that loop.
>
> So when we get here we know pinned == entries, and if pinned is zero
> it's because we took the (dev_hpa != MM_IOMMU_TABLE_INVALID_HPA) case at
> the start of the function to get here.
>
> Or do you think that's too subtle to rely on?
I had 4 choices:
1. for (;i < pinned;)
2. if (dev_hpa == MM_IOMMU_TABLE_INVALID_HPA) (dev_hpa is a function
parameter)
3. if (mem->dev_hpa == MM_IOMMU_TABLE_INVALID_HPA)
4. if (mem->hpages)
The function is already ugly. 3) seemed as the most obvious way of
telling what is going on here: "we have just initialized @mem and it is
not for a device memory, lets finish the initialization".
I could rearrange the code even more but since there is no NVLink3
coming ever, I'd avoid changing it more than necessary. Thanks,
>
> cheers
>
>> + struct page *page = mem->hpages[i];
>> +
>> + if ((mem->pageshift > PAGE_SHIFT) && PageHuge(page))
>> + pageshift = page_shift(compound_head(page));
>> + mem->pageshift = min(mem->pageshift, pageshift);
>> + /*
>> + * We don't need struct page reference any more, switch
>> + * to physical address.
>> + */
>> + mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
>> + }
>> + }
>> +
>> list_add_rcu(&mem->next, &mm->context.iommu_group_mem_list);
>>
>> mutex_unlock(&mem_list_mutex);
>> --
>> 2.17.1
--
Alexey
